1. Field of the Invention
The present invention relates to a screw set for use in a twin-screw extruder for kneading materials to be kneaded, such as rubber and plastic.
2. Description of the Related Art
Generally, a twin-screw extruder is constructed by providing a pair of screw sets in a barrel having a chamber extending from one side to the other side, each of the screw sets being a combination of kneading segments, such as kneading rotors and kneading disks, and screw segments. A kneaded product is continuously produced by a series of operations of charging a material to be kneaded, such as rubber and plastic, into the barrel through an inlet at one side, kneading the charged material by the kneading segments while feeding the material toward the other side with the rotation of the screw sets, thereby forming a kneaded product in a desired state of kneading, and extruding the kneaded product to the outside from a tip end of the extruder (see FIGS. 1A and 1B for general construction).
The above-mentioned kneading rotors have tip clearances greater than those of the kneading disks, and therefore are advantageous in that a target material of kneading, which comprises a material to be kneaded and a kneaded material, can be kneaded at relatively low temperatures. However, since the greater tip clearances render the target material of kneading to adhere to an inner wall surface of the barrel, the kneading rotors have disadvantages that a trouble, such as a deterioration of the target material of kneading due to residence for a long time in the barrel or a damage of the screw set due to a load partially imposed on the kneading rotors, is more likely to occur.
Recently, in view of the above-mentioned disadvantages, there has been proposed a twin-screw extruder comprising, as shown in FIG. 11, a pair of kneading rotors 51 and 51 each having kneading blades 51a and 51b that are formed to have contour shapes allowing the kneading rotors 51 and 51 to completely mesh with each other when a pair of left and right screw sets are arranged as shown, particular one 51 a of the kneading blades being formed to provide a smaller tip clearance (see Japanese Unexamined Patent Application Publication No. 10-264148). with this construction, the target material of kneading, which has adhered to the inner wall surface of the barrel, can be scrapped off by the kneading blade 51a having the smaller tip clearance, while ensuring advantages that one of the kneading rotors 51 and 51 meshing with each other functions to prevent the target material of kneading from adhering to the other, and the kneading blade 51b having a greater tip clearance serves to knead and disperse the target material of kneading into a satisfactory state, thereby preventing an excessive temperature rise.
In the conventional construction described above, however, since the kneading blades 51a and 51b are formed so as to allow the kneading rotors 51 and 51 to completely mesh with each other, the sectional shapes of the kneading rotors 51 and 51 are not in match with those of the kneading disks and screws. More specifically, in FIG. 11, the axis-to-axis distance between the left and right screw sets is constant, and the left and right kneading rotors 51 and 51 are completely meshed with each other. Looking at a certain section, therefore, when one kneading blade of the right kneading rotor, which has the greater tip clearance, is in a horizontal position, a relatively thin belly portion of the opposite left kneading rotor is positioned in an opposing relation. On the contrary, when the other kneading blade of the right kneading rotor, which has the smaller tip clearance, is in a horizontal position, a relatively thick belly portion of the opposite left kneading rotor is positioned in an opposing relation. in the arrangement that the left and right kneading rotors are completely meshed with each other and the kneading blades of each rotor are formed to provide different tip clearances, therefore, assuming the screw sets to be cut at a certain section, the sectional shapes of the left and right kneading rotors are asymmetrical and hence not the same on the left and right sides. Stated otherwise, looking at one screw set, the sectional shape of the kneading rotor is changed in the axial direction thereof.
Supposing now, as described above, that the sectional shapes of the left and right kneading rotors are changed in the axial direction thereof so as to allow both the kneading rotors to completely mesh with each other, i.e., that the left and right kneading rotors are formed to have the sectional shapes asymmetrical on the left and right sides, if two adjacent segments in the axial direction of one screw set have different sectional shapes from each other, two adjacent segments in the axial direction of the other screw set, which are respectively in mesh with the former two adjacent segments, also have different sectional shapes from each other. This means that, among four segments of the left and right screw sets, pairs of which are opposed in the transverse direction and adjacent to each other in the axial direction, each pair of segments in an obliquely opposing relation are not properly meshed with each other. In other words, the segments of both the screw sets, which are in an obliquely opposing relation and should not be meshed with each other in a proper setup condition, are positioned side by side in the transverse direction at connecting portions between the axially adjacent segments of the screw sets upon a slight shift in the axial direction, and then interfere with each other at the connecting portions because of the left and right kneading rotors having different sectional shapes. Thus, if two adjacent segments of each screw set in the axial direction have sectional shapes different at a connecting portion between the segments, the left and right rotors interfere with each other at the connecting portions upon a slight shift (relative movement) of the left and right rotors in the axial direction thereof Therefore, the related art requires a spacer to be inserted in the connecting portion between every two axially adjacent segments of each screw set. The spacer has a thickness enough to absorb such a slight rotor shift in the axial direction, and also has a sectional shape selected such that the spacers arranged in an opposing relation are prevented from interfering with each other due to their sectional shapes.
In the case of inserting such spacers, the spacers produce gaps between the segments and hence give rise to a problem that, since the target material of kneading tend to reside in the gaps for a long time and deteriorate, a degradation in quality of the kneaded product caused by the deterioration of the target material of kneading cannot be sufficiently prevented.
Furthermore, when the tip clearance of the rotor segment is changed at short intervals in the axial direction for reducing a load partially imposed on the rotor segment, a rotor segment having a small axial length must be prepared. This leads to another problem that the cost of manufacturing the rotor segment is increased and the assembly operation takes larger amounts of labor and time.